The present invention relates to peptides which exhibit potent anti-retroviral activity. The peptides of the invention are derived from regions of viral fusion proteins referred to as HR1 and HR2. In particular, the invention relates to peptides referred to herein as DP107 and DP178 which comprise amino acid sequences corresponding to sequences found in the HR1 and HR2 regions, respectively of the HIV-1.sub.LAI gp41 protein. The invention further relates to "DP107-like" and "DP178-like" peptides that are derived from HR1 and HR2 regions, respectively, of other proteins, including DP107-like and DP178-like peptides derived from the HR1 and HR2 regions of the F1 subunit of the respiratory syncytial virus fusion protein.